Preparation of electrodes for electrolysis of aqueous solutions by the mercury process



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It is known that the support for the mobile cathode in horizontalmercury cells is constituted in a large number of cases by a steel oriron plate. In the case of mercury cells of the vertical type, with orwithout a support for the mobile cathode, iron is usedpracticallyexclusively. It is further known that the metal support must beperfectly amalgamated in order to ensure the best possible circulationand distribution of the mercury and in order to avoid prematurecorrosion of the metal support by the electrolysis. The Wetting of themetallic surface thus attains considerable importance especially whenthe cell is put into service. It has been proposed to cover the cathodesupport (German Patent No. 692,- 954), especially if it is of iron, witha very thin layer of another metal such as tin, copper, lead or thelike. This metal deposit has as its object the favouring of the wettingby the mercury, the met-a1 deposited having the well known property ofeasy amalgamation.

This process is, however, not without disadvantages. The metals thuscarried dissolve in the mercury with the result that the iron supportsdeteriorate rather quickly and that after the cell has been stopped itis necessary to renew the amalgamatable covering; furthermore, the metalwhich has passed into solution in the mercury is carried by the lattertowards the decomposition apparatus for the amalgamation and it depositson the graphite of the cathodes of this apparatus and thus rapidlyreduces their activity. Moreover the couples formed by the coveringmetals and the iron which they cover can bring about corrosion in thelatter. The object of the present invention is to avoid thesedisadvantages. The invention is based on the observation that thequality of the iron used as supporting metal for the mercury cathode ofthe electrolytic cell has a marked influence on the ease of amalgamationof this support; thus, electrolytic iron amalgamates more easily andmaintains its amalgamation far longer than in the case of ordinary ironor steel.

The invention consists in a process for making supports for mobilecathodes of electrolytic cells of the vertical type which ischaracterised in that there is electrolytically deposited on a metallicplate, preferably of iron, a layer of pure iron, this layer beingseveral tenths of millimetre in thickness. The added electrolytic ironof which the purity may be very high does not dissolve in the mercury asthe readily amalgamated metals do. This addition to the base metal iscarried out once and for all since the life of the layer is very longcompared with that of the other metals referred to. Electrolyticdeposition of iron, correctly carried out, provides a very finelyroughened surface which, particularly in the case of the vertical mobilemercury cathode cells, retains mercury and amalgam, thus assuringefficient and regular operation of the apparatus.

To produce such an addition of iron for example with a thickness of upto 0.9 millimetre a known bath is used containing for example 300 gramsof FeCl .2l-I O and 150 grams of CaCl per litre of solution, having aatent C pH value of 1.5 to 2 with or without further additions; thecurrent density being between 4 and 4.5 amperes per square decimeter ofcathode surface and the temperature being of the order to C.

The invention is not limited to the formation of a mercury cathodesupport with a uniformly polished surface. It comprises likewise themanufacture of grooved supports and embossed supports such as thosewhich form the object of U.S. patent application 479,746 filed January4, 1955, in the names of Charles Deprez and lean Clement, now Patent No.2,849,393.

According to a further feature of the invention a network of lines istraced on the supporting plate with the aid of an insulating varnish andan electrolytic deposit of iron is obtained on the surface not coveredby varnish, up to a thickness of some tenths of millimeter after whichthe varnish is dissolved away from the grooves thus formed.

It is clear that the varnish must be resistant to the physical andchemical influences to which it will be submitted.

In the same way one can obtain embossings of any shape constituted bythe added electrolytic iron.

The supports thus formed present, apart from the advantage 'alreadystated, that of a lower cost of replacement than in the case of agrooved cathode prepared by mechanical means. Furthermore, it ispossible to obtain a groove-design as complicated as can be desiredwhereas this is difficult to achieve by mechanical means. Likewise onecan treat surfaces of any size or shape.

The deposits of electrolytic iron can be obtained in the electrolyticcell itself. To this end it is suflicient to replace the graphite anodesby iron anodes and to replace the electrolyte by an appropriate solutionhaving the composition indicated above. Operating in this way permitseconomy in the use and with special respect to special installationswhich are generally undesirable.

I claim:

1. In an electrolytic cell having a mobile mercury cathode, a supportfor said mercury comprising a metallic base plate having upon itsmercury-contacting surface an electro-deposited coating of soft ironhaving a thickness up to 0.9

2. The apparatus of claim 1, wherein said electrodeposited coatingcovers limited areas of said base plate, thereby forming a groovedpattern on said plate.

References Cited in the file of this patent UNITED STATES PATENTS with aMercury Cathode, Field Information Agency Technical, June 17, 1946, page7.

1. IN AN ELECTROLYTIC CELL HAVING A MOBILE MERCURY CATHODE, A SUPPORTFOR SAID MERCURY COMPRISING A METALLIC BASE PLATE HAVING UPON ITSMERCURY-CONTACTING SURFACE AN ELECTRO-DEPOSITED COATING OF SOFT IRONHAVING A THICKNESS UP TO 0.9 MM.